1. Field of the Invention
The present invention relates to an anti-skid control system for use in an automotive vehicle, and more particularly to an anti-skid control system for controlling braking force applied to road wheels in braking operation to prevent the road wheels from being locked.
2. Description of the Prior Art
It is known that the vehicle stability is influenced detrimentally depending upon the road surface condition, when road wheels are locked in abrupt braking operation. In order to prevent the road wheels from being locked, therefore, there has been employed an anti-skid control system which controls the braking force by decreasing or recovering a hydraulic braking pressure supplied to a wheel brake cylinder
In general, the anti-skid control system is arranged to detect a rotational speed of each road wheel or a wheel speed, calculate an estimated vehicle speed on the basis of the wheel speed, and calculate a reference speed on the basis of the estimated vehicle speed The system is arranged to compare the wheel speed of each road wheel with the reference speed, and control the hydraulic braking pressure in the wheel brake cylinder mounted on each road wheel, in response to a result of comparison.
The above-described reference speed is provided for setting a threshold level to initiate decreasing the hydraulic braking pressure, and generally set to a value obtained by subtracting a certain speed from a certain rate of the estimated vehicle speed In the anti-skid control method disclosed in Japanese Patent Laid-open Publication No.48-41173, for example, the above-described value corresponds to a set voltage defined as a voltage obtained by subtracting a certain voltage from a certain rate of a voltage corresponding to a wheel speed. The set voltage, or the reference speed is set to such a value as described in the above publication, so as to avoid an erroneous operation which may be caused in a low speed area or the like, if the estimated vehicle speed is directly compared with the wheel speed as a reference speed.
However, in the anti-skid control operation based upon the above-described reference speed, when the hydraulic braking pressure is gradually applied to the wheel brake cylinder, to be slightly higher than the hydraulic pressure for causing a road wheel to be locked, there may exist such a condition that the wheel speed Vw will be less than the estimated vehicle speed Vs, but greater than the estimated vehicle speed (Vs), so that it will be unable to initiate decreasing the hydraulic braking pressure Accordingly, if the above condition continues, the road wheel will be locked without decreasing the hydraulic braking pressure in the wheel brake cylinder.
FIG. 12 shows an operation of the prior anti-skid control system in the above condition. In the case where the wheel speed Vw is maintained to be less than the estimated vehicle speed Vs, and greater than a reference speed Vc (=K5.multidot.Vs-K6), the hydraulic braking pressure in the wheel brake cylinder is gradually increased without being decreased, so that the wheel speed Vw is getting remote from the actual vehicle speed Vr to have the road wheel locked. This will possibly happen in the case where the anti-skid operation is performed while the vehicle is running on a road of a low coefficient of friction having a characteristic of a relatively flat .mu.-S curve which indicates a relationship between the coefficient of friction and the slip rate, especially in the case where the estimated vehicle speed Vs is calculated on the basis of the wheel speed of a single road wheel. As to the values K5, K6 for setting the reference speed Vc, the value K5 indicates a certain rate (e.g., 0.95), and the value K6 indicates a certain speed (e.g., 2 km/h).
On the contrary, if the reference speed Vc was set to a greater value to be close to the estimated vehicle speed Vs, the problem raised in the above publication could not be solved. Thus, the hydraulic braking pressure in the wheel brake cylinder would be excessively decreased due to a peculiar change in the wheel speed, or the like, so that the braking distance is likely to be extended.